1. Field of the Invention
The present invention relates to an ion source system, and more particularly, to an improved double chamber ion source that effectively provides heavy gas molecular ions through charge exchange.
2. Description of the Related Art
Numerous semiconductor manufacturing processes employ ion implantation for forming a p-n junction by adding dopants (impurities), such as boron (B) and phosphorus (P) to a semiconductor substrate. Ion implantation makes it possible to accurately control the concentration and depth of impurities to be diffused into a target spot on the semiconductor substrate.
Typically, an ion implanter includes an ion source that ionizes an atom or molecule of the material to be implanted. The generated ions are accelerated to form an ion beam that is directed toward a target, such as a silicon chip or wafer, and impacts a desired area or pattern on the target. The entire operation is carried out in a high vacuum.
The trend in semiconductor devices is to become smaller and thinner. As such, these smaller and thinner requirements challenge the ability of present systems to produce high dose ion beams with the low energy required to implant a high concentration of ions at a shallow depth in the semiconductor device.
Ion current (current density x area) and beam energy are the fundamental process variables for the implant step. Ion dose and implant range are the resultant device variables. Ion dose relates to the concentration of implanted ions in a semiconductor material. Moreover, the energy of the ion beam determines the depth of the implanted ions before the activation anneal step. The dose rate and, therefore, the process time is proportional to the ion current. Ideally, ion dose rate and beam energy would be independent process variables. This is somewhat true for high energy beams used for deep implants. However, low energy ion beams that are used for shallow implants, but, for standard dopant atomic ions at low beam energies, have ion currents that are constrained by physics limitations associated with extraction and transport losses.
Present ion implanters operate best at energies from about 10 keV to about 2 MeV. Shallower implant of ions will require similar beam currents as present implanters, but at much lower energies, e.g., from about 2 keV down to hundreds of eV. However, as beam energy decreases to accommodate thinner devices, beam transport of standard ions, defined as dopants, such as boron (B+), arsenic (As+) and phosphorus (P+), becomes inefficient due to beam space charge. Beam space charge may be defined as the repelling of like charges in the ion beam causing an expansion of the ion beam during transport to the target. As a result, beam transmission is greatly reduced when the energy level is reduced.
The possibility of producing useful currents of a heavy gas phase molecular ion offers significant advantages over ion source material presently used in implanters. For example, using the heavy gas molecular ion, decaborane ion (B10H14+), which has ten boron atoms has advantages for low energy, high current dopant beam transport. First, the energy for each individual boron nucleus is one tenth the energy of the ion, making it possible to extract and transport at approximately ten times the beam energy of boron ions. For example, a 10 keV beam of B10H14+ would deliver dopant at less than 1 keV per boron atom. Second, the dopant nucleon current is ten times the ion current. As such, only about 1 mA of B10H14+ is needed to deliver 10 mA of boron.
Thus, it would be advantageous to provide an ion source that produces heavy ions with multiple dopant atoms per ion, at a sufficient dose (current density) to be effective as an ion in an implanter system, especially for shallow depths.
However, attempts to produce ions of heavy dopant molecules in standard single ion sources have generally been unsuccessful. Several mechanisms can be present in single chamber sources that tend to breakup the large molecules, for example, collisions with energetic electrons, collisions with hot gas particles, contact with hot components such as filament or walls, plasma radiation, and filament radiation. Each of these can cause heavy molecules to break apart before they are ionized and/or implanted on the target substrate. Furthermore, even if the heavy molecule is not disassociated, charge transfer may occur in the ion beam thereby neutralizing the ion.
Charge transfer or electron charge exchange, is a result of collisions between a neutral particle and a charged ion and is considered a loss mechanism in ion beam systems. The simplest kind of charge transfer involves a collision between a neutral particle and a singly charged ion:
xe2x80x83A++B A+B+
where A, B denote neutral particles in the ground state, and the superscript xe2x80x9c+xe2x80x9d indicates a single positive charge state. In this case, ion B+ is created by an electron transfer from atom B to ion A+.
Charge transfer can cause significantly different energy patterns in the ion beam dependent upon the region wherein charge transfer occurs. For instance, transfer product ions formed in the acceleration gap of an ion implanter experience less than full acceleration potential, and thus differ in energy from ions accelerated through the full gap. As such, charge transfer is usually undesirable because it reduces the current density of the desired ion at the desired impact energy.
Accordingly, it would be a significant advance in the art of ion implantation to provide an ion source that generates heavy gas molecular ions through a charge transfer mechanism at a sufficient current density to be effective in an implanter system, while eliminating or significantly reducing mechanisms present in standard plasma sources that breakup heavy molecules and/or molecular ions.
It is therefore a principal object of the present invention to provide an improved charge exchange molecular ion source.
Another object of the present invention is to provide an ion source system that provides a stream of ions having sufficient density to effectively coat a target substrate.
Still another object of the present invention is to provide a high energy ion beam that provides shallow implantation of heavy molecular ions.
Yet another object of the present invention provides heavy gas molecules at a sufficient current density to be effective in an implanter system.
A further object of the present invention is to improve the transport efficiency for a low energy ion beam for shallow junction implant by reducing the beam losses caused by beam space charge.
These and further objects are accomplished by improved ion sources disclosed herein.
One aspect of the present invention relates generally to an ion source for an implantation system that utilizes charge transfer to enhance production of a desired molecular ion species. The ion source contains a minimum of two regions separated by a physical barrier and utilizes charge exchange for production of a desired molecular ion species. The physical barrier serves to reduce the effect of mechanisms that are destructive for heavy molecules and ions, such as, collisions with energetic electrons, collisions with hot gas particles, contact with hot components such as filament or walls, thermal energy transfer, plasma radiation, and filament radiation. The barrier has at least one aperture that allows ions to flow from the plasma chamber into the charge transfer chamber. The barrier also serves to inhibit destructive mechanisms.
The essential elements of the present invention include at least one plasma chamber for production of ions of a first species, a divider structure, and at least one charge exchange region where ions of the first species, generated in the plasma chamber, undergo charge exchange or transfer with a reactant atom or molecule to produce ions of a second species.
Another aspect of the present invention provides for an ion source comprising at least one plasma generating chamber having a plasma generating means for generating ions. Adjacent to the plasma generating chamber is at least one charge exchange chamber, wherein a molecular species is ionized by electron transfer from a first ionized species extracted from the plasma generating chamber. The charge exchange chamber further comprises a magnetic shield to shield the second ion species, generated therein, from an exterior magnetic field. This embodiment may further comprise means to cool the magnetic shield. A magnetic shield can be created by placement of magnetic material, such as, iron, or, by electrical means, such as an electrical circuit that creates a magnetic field to modify the local magnetic field from the plasma source magnet.
Yet another aspect of the present invention provides for an ion source comprising at least one plasma generating chamber and at least one charge exchange chamber, the charge exchange chamber is separated from the plasma generating chamber by a divider structure having a plurality of apertures for introducing ions generated in the plasma generating chamber into the charge exchange chamber, the apertures having a configuration that reduces back-migration of ions and gas from the charge exchange chamber to the plasma generating chamber.
A further aspect of the present invention provides for an ion source comprising at least one plasma generating chamber and at least one charge exchange chamber, the charge exchange chamber is separated from the plasma generating chamber by a divider structure having at least one aperture. A heat shield is positioned between the charge exchange chamber and plasma generating chamber to reduce heat conduction from the plasma generating chamber.
A still further aspect of the present invention provides for an ion source comprising at least one plasma generating chamber and at least one charge exchange chamber, the charge exchange chamber is separated from the plasma generating chamber by a divider structure having at least one aperture for emission of ions generated in the plasma generating chamber into the charge exchange chamber. The plasma chamber farther comprises at least one magnet oriented to establish magnetic field components transverse to the direction of travel of ions from the ion source region to the charge exchange region. The magnet may further comprise a yoke to return magnetic flux and minimize magnetic flux leakage into the charge exchange chamber.
Still another aspect of the present invention provides for an ion source comprising at least one plasma generating chamber and at least one charge exchange chamber, the charge exchange chamber is separated from the plasma generating chamber by a divider structure having at least one aperture for emission of ions generated in the plasma generating chamber into the charge exchange chamber, the plasma chamber further comprising at least one magnet oriented to establish a magnetic field having field components in the direction of travel of ions from the plasma generating chamber to the charge exchange chamber, thereby increasing the ion transfer efficiency from the plasma chamber into the charge exchange chamber.
Other aspects, features and embodiments of the present invention will be more fully apparent from the ensuing disclosure and appended claims.